Here’s how Jonathan Kirsch describes waterboarding in his excellent and troubling 2008 Harper-Collins book, The Grand Inquisitor’s Manual: A History of Terror in the Name of God (p. 4):

Among the first and favorite forms of torture used by medieval inquisitors was the so-called ordeal by water, that is, pouring water down a victim’s throat to simulate the sensation of drowning and thereby extract a confession. As far as the Inquisition was concerned, the ordeal by water was an ideal method of interrogation: it required only a bucket of water and a funnel, it left no telltale marks and no bloody mess to clean up, and yet it produced such agony and terror that the victim would readily tell the torturer whatever he wanted to hear. That’s why the ordeal was favored not only by the medieval inquisitors but also by their successors in the Gestapo and the Soviet secret police. And the same form of torture is still in use today, although we are asked by its modern users and defenders to call it “waterboarding.”